1. Field of the Invention
This invention relates to an optical system for use in a silver halide film camera and a digital still camera, and in particular, to an optical system having a fiber optical plate.
2. Description of Related Art
An optical element called a fiber optical plate (hereinafter abbreviated to FOP) has been known as an image transmitting element using an optical fiber. This optical element is the same in principle as an image guide and has a compact design such that its length can be as short as several tens of millimeters. Such FOPs are of two types; one, in which an image formed on the entrance surface of the optical element is transmitted to the exit surface thereof with unity (1.times.) magnification, and the other, in which a single fiber constituting the FOP is shaped into a taper-like form to magnify the image formed on the entrance surface while transmitting the image to the exit surface. In particular, a taper type FOP of the latter has severalfold magnification and, in contrast with the case of a conventional technique of changing an image magnification by a refractive function of a lens, can be materially reduced in size.
In recent years, digital cameras have become more popular. To meet the needs of this industry, there is competition to achieve higher pixel density and smaller image pickup devices. In order to make sure of the photographing area of the digital camera, a liquid crystal display or an optical finder is utilized. The optical finder has the benefit of requiring less battery than the liquid crystal display. In most cases, however, the optical finder is constructed to be independent of a photographic lens, and thus the production of parallax is unavoidable in the optical finder. This problem becomes more severe as an object distance is diminished or as a variable magnification ratio is increased.
Consequently, a camera using a TTL type optical finder is proposed. This optical finder is known as a Keplerian real image mode finder, such as that used in a silver halide film camera, or a relay real image mode finder. In either case, if an imager is downsized, the intermediate image of a finder system will diminish in size. However, since a finder magnification and a field angle which an observer desires remain unchanged, magnification for magnifying the intermediate image must be increased. In this way, the power load of an eyepiece is increased, resulting in oversizing of the eyepiece and a rise in cost due to the necessary aberration and an increase of the number of lenses.
In an optical system with a film of relatively large size as in the silver halide film camera, an image to be photographed and an intermediate image observed through a finder are enlarged, and thus a photographing optical system and a finder optical system become oversized, which is unfavorable.